Electrical circuits requiring high power handling capability (&gt;20 watts) while operating at high frequencies such as radio frequencies (500 MHz), S-band (3 GHz) and X-band (10 GHz) have in recent years become more prevalent. Because of the increase in high power, high frequency circuits there has been a corresponding increase in demand for transistors which are capable of reliably operating at radio frequencies and above while still being capable of handling higher power loads. Previously, bipolar transistors and power metal-oxide semiconductor field effect transistors (MOSFETs) have been used for high power applications but the power handling capability of such devices is generally limited at higher operating frequencies. Junction field-effect transistors (JFETs) were commonly used for high frequency applications but the power handling capability of previously known JFETs is limited.
Recently, metal-semiconductor field effect transistors (MESFETs) have been developed for high frequency applications. The MESFET construction is preferable for high frequency applications because only majority carriers carry current. The MESFET design is preferred over current MOSFET designs because the reduced gate capacitance permits faster switching times of the gate input. Therefore, although all field-effect transistors utilize only majority carriers to carry current, the Schottky gate structure of the MESFET makes the MESFET more desirable for high frequency applications.
In addition to the type of structure--and perhaps more fundamentally--the characteristics of the semiconductor material from which a transistor is formed also affects the operating parameters. Of the characteristics which affect a transistors operating parameters, the electron mobility, saturated electron drift velocity, electric breakdown field and thermal conductivity have the greatest effect on a transistor's high frequency and high power characteristics.
Electron mobility describes how strongly the motion of an electron is influenced by the application of an electric field. Thus if a particular semiconductor material has a high electron mobility, a lesser electric field is required to move the same number of electrons than is required in a material that has a lower electron mobility. In the past, semiconductor materials which have a high electron mobility were preferred because more current could be developed with a lesser field, resulting in faster response times when a field is applied.
Saturated electron drift velocity is the maximum velocity which an electron can obtain in the semiconductor material. Materials with higher saturated electron drift velocities are preferred for high frequency applications because the higher velocity translates to shorter times from source to drain.
Electric breakdown field is the field strength at which breakdown of the Schottky junction and the current through the gate of the device suddenly increases. A high electric breakdown field material is preferred for high power, high frequency transistors because larger electric fields can be supported by a given dimension of material. Larger electric fields allow for faster transients as the electrons can be accelerated more quickly by larger electric fields than by smaller.
Thermal conductivity is the ability of the semiconductor material to dissipate heat. In typical operations, all transistors generate heat. In turn, high power and high frequency transistors usually generate larger amounts of heat than small signal transistors. As the temperature of the semiconductor material increases, the junction leakage currents increase and the current through the field effect transistor decreases due to a decrease in carrier mobility with an increase in temperature. Therefore, if the heat is dissipated from the semiconductor the material will remain at a lower temperature and be capable of carrying larger currents with lower leakage currents.
In the past, most high frequency MESFETs have been manufactured of n-type III-V compounds, such as gallium arsenide (GaAs) because of their high electron mobilities. Although these devices provided increased operating frequencies and moderately increased power handling capability, the relatively low breakdown voltage and the lower thermal conductivity of these materials have limited their usefulness in high power applications.
Silicon carbide (SiC) has been known for many years to have excellent physical and electronic properties which should theoretically allow production of electronic devices that can operate at higher temperatures, higher power and higher frequency than devices produced from silicon (Si) or GaAs. The high electric breakdown field of about 4.times.10.sup.6 V/cm, high saturated electron drift velocity of about 2.0.times.10.sup.7 cm/sec and high thermal conductivity of about 4.9 W/cm-K indicate that SiC would be suitable for high frequency, high power applications. Unfortunately, difficulty in manufacturing has limited the usefulness of SiC for high power and high frequency applications.
Recently, MESFETs having channel layers of silicon carbide have been produced on silicon substrates (see U.S. Pat. Nos. 4,762,806 to Suzuki et al and 4,757,028 to Kondoh et al). Because the semiconductor layers of a MESFET are epitaxial, the layer upon which each epitaxial layer is grown affects the characteristics of the device. Thus, a SiC epitaxial layer grown on a Si substrate has different electrical and thermal characteristics then a SiC epitaxial layer grown on a different substrate. Although the SiC on Si substrate devices described in U.S. Pat. Nos. 4,762,806 and 4,757,028 may have exhibited improved thermal characteristics, the use of a Si substrate limits the ability of such devices to dissipate heat. Furthermore, the growth of SiC on Si results in defects in the epitaxial layers which result in high leakage current when the device is in operation.
Other MESFETs have been developed using SiC substrates. U.S. patent application Ser. No. 07/540,488 filed Jun. 19, 1990 and now abandoned, the disclosure of which is incorporated entirely herein by reference, describes a SiC MESFET having epitaxial layers of SiC grown on a SiC substrate. These devices exhibited improved thermal characteristics over previous devices because of the improved crystal quality of the epitaxial layers grown on SiC substrates. However, to obtain high power and high frequency, one must overcome the limitations of SiC's lower electron mobility. As such, additional development of the structure of the MESFET is required.